12v battery array w/disparate battery sizing

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I have a remote cabin in the woods that gets sporadic use. I installed a small panel (75w iirc) on the roof that feeds a Morningstar charge controller a few years back - model number for both the panel and CC are around here somewhere but I can't find them at the moment. The CC feeds 2 autozone/kragen style deep cycle batteries.

We run a small 150 watt inverter off of the batteries for 2 ceiling fans, stove clock, TV, stereo, etc in the cabin. batteries usually run out well before the weekend is over if we run anything for too long.

a few months ago I ended up with 3 more 12v deep cycle batteries that came along in a boat that I bought, 2 of the 3 are EverStart 115amp hour and matching in size, the third is a trojan deep cycle of some sort that doesn't have a label.

I'd like to haul these other 3 batteries out to the cabin and let the small panel on the roof put a charge in them as part of a battery array, but the mismatched sizing on the batteries makes me think that I need some sort of isolator so that I can charge these off the panel in 3 isolated groups - the current 2 batteries, the 2 matching EverStarts, and the Trojan.

I'm out of my knowledge zone here - is there some sort of product available that I should be looking for that will do what I want? or can I build some sort of circuit? Looking for a point in the right direction, hope the info above makes sense. Appreciate any help.

Mike
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: 12v battery array w/disparate battery sizing

    This is not a promising plan. The reason being that anything which isolates the batteries from each other, such as a diode, will also isolate them from the charge controller which needs to be able to 'read' the batteries to know if they're charged. Plus, you don't know if the controller can handle that many batteries. Finally, I'm absolutely certain that a 75 Watt panel can't charge that cumulative Amp/hr load - it would handle about 125 Amp/hrs at best.

    My suggestion is to keep the batteries separate, charge & use them in rotation until you can get a good set-up going. You can start that by determining your actual usage rate with a Kill-A-Watt meter (about $30).

    Chances are these batteries are all of the "RV/Marine" type - not the first choice for solar power.

    But hey, you have to start somewhere right? :D
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
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    Re: 12v battery array w/disparate battery sizing

    Read through the "off grid" section of thread on this forum. If you read a bunch of threads you will begin to know what you need to do.

    The reality is it is best to start with your estimated loads, and work backwards to sizing battery banks and Pv. A few important things to remember. The first is that cycling batteries more than ~1/2 (50% state of charge/depth of discharge) is a recipe for killing them quickly. Some (including me) suggest not going lower than 80% soc or 20% dod for longest battery life. Said batteries should also be returned to 100% soc within about 3 days for longest battery life.

    Second, small battery based solar systems will (in very rough numbers) put out about 50% of rated name plate rating. To find how much solar you can reasonably expect to get from X panel do this quick calc. Lets say your panel is 100 watts, 100/.5*4=200 watt/hours per day. (The 4 is the number of hours of GOOD sun to expect, seldom much more than 4 on average.)

    The reality as Marc suggests,, if you find your loading you will probably be surprised as to how much you really are using compared to what you think you are using. We live off grid and we use ~3-400wh/day with no TV, no micro, no electrical heating appliances etc.

    I suggest you also read the following links:http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries
    http://www.batteryfaq.org/

    Welcome to the site, good luck and don't hesitate to ask more questions. Remember to avoid the biggest potential pitfall,,, that is , "ready, fire, aim!

    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,440 admin
    #4
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    Re: 12v battery array w/disparate battery sizing

    And adding batteries will not resolve the the issue of not enough solar panels to keep up with your loads.

    Repetitive deep cycling your batteries (below 50%) takes life away.

    Keeping the flooded cell batteries below 75% State of Charge (25% discharged) for days+ at a time causes the sulfates to harden and quickly reduces the capacity of the batteries.

    Get a Kill-A-Watt meter to measure your AC loads--and work backwards into how much DC power you are using. Then how much DC power your panels (+backup genset?) is supplying.

    A 75 watt panel, realistically, supplies around (assuming 5 hours of "good" summer sun) (please note--the numbers below are just rough rules of thumb to give you a rough idea of your system capacity--we can go into much more detail if you need to. It is very possible that you would get 10-20% more power than I have listed below on "good" days--or a lot less depending on a lot of other issues):

    5 hours of sun * 75 watt * 50% overall efficiency = 187.5 Watt*Hours per day

    Depending on what your AC loads are, that could easily be used in just a few hours with your current loads. Say you have 90 watts of loads; to figure out how long they will run:

    187.5 Watt*Hours / 90 Watts = 2.1 hours per day

    So, you take some solar power + stored power from your battery bank and every day the batteries get more and more discharged... In a few days--they are near dead and you have to cut power to charge them back up (and/or use a genset).

    Do you have a good hydrometer to measure the battery bank state of charge?

    At some point, if you grow your system, you will probably want a battery meter which can read out State of Charge in percent. Darn near the equivalent of a gas gauge for your fuel tank.

    You can play some games, like using AGM batteries (deeper cycling does not cause sulfates to harden, more efficient charging 90% vs 80% losses for flooded cell, etc.)... But AGMs are 2x or more as expensive--and if you want to stay in your cabin for more than 2-3 days a week--you still must make for deficit charging.

    You can make up the charging with 1. more solar panels, or 2. a small Honda eu2000i or eu1000i genset. Very fuel efficient and quite--plus useful if more power is needed (power saw for working on the cabin, etc.).

    The issue is that solar panels give you the best return for your money if the power is used ~9 months out of the year. If the power is only used on summer weekends--many times a fuel efficient/quiet genset is really a better fit--and just use enough solar panels for trickle charging when you are not there. And you still have the batteries for a quiet evening at the cabin.

    Lastly, your inverter may have a fair amount of load just to operate--sometimes up to 8 watts. The MorningStar 12 volt 300 watt True Sine inverter is a nice piece of hardware for small AC loads in cabins. If you can justify the price of the True Sine Wave inverter--I would really suggest it--much fewer issues with respect to the very inexpensive MSW (modified square/sine wave) inverters.

    Kill-A-Watt
    Battery Monitor
    Battery FAQ
    Precision Hydrometer
    Morningstar SureSine, 300 Watt Sine Wave Inverter 115VAC (not cheap)
    All About Inverters
    Choosing an inverter for water pumping

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: 12v battery array w/disparate battery sizing

    Semantic note: "Get a Kill-A-Watt meter and work backwards ..."

    In my opinion that's working forwards. Backwards is the way most people do it: run out and buy a bunch of stuff before they know what they need then try to make it work. Like this:

    "I bought a Harbor Freight 45 Watt solar power system. Why won't it run my 1500 Watt microwave?"

    I'm being nastily sarcastic I know. But determining your loads is where to start. Everybody ought to do this, whether they're planning on 'going solar' or not. It's Step 1.

    Steps 2, 3, 4, and possibly 5 are: take conservation measures.

    Then we get to the alternate energy sources. :D
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,440 admin
    #6
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    Re: 12v battery array w/disparate battery sizing
    Cariboocoot wrote: »
    Semantic note: "Get a Kill-A-Watt meter and work backwards ..."

    OK--I am going to edit your post like NY Times' Maureen Dowd does:
    In my opinion that's working ... ackwards ... [M]ost people do it: run out and buy a bunch of stuff .. they know what they need then ... make it work. Like this:

    "I bought a Harbor Freight 45 Watt solar power system. Why ... run my 1500 Watt microwave..."

    ... I know ... [e]verybody ought to do this... :D
    Now the ol'Coot agrees with me. :p

    -Bill "LMAO ROTF" B. ;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: 12v battery array w/disparate battery sizing

    I am also now very glad I don't read the NY Times. :p
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #8
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    Re: 12v battery array w/disparate battery sizing

    I spent most my life backwards!

    Tony
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: 12v battery array w/disparate battery sizing
    icarus wrote: »
    I spent most my life backwards!

    Tony

    Sheesh! You Canadians!

    Uh, wait a minute ... :blush:
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #10
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    Re: 12v battery array w/disparate battery sizing

    Back to the original question,,, (Or forward to it!)

    The OP writes "
    "We run a small 150 watt inverter off of the batteries for 2 ceiling fans, stove clock, TV, stereo, etc in the cabin. batteries usually run out well before the weekend is over if we run anything for too long."

    2 ceiling fans maybe 25 watt each, Stove clock 5 watts (Get a battery or wind up) stereo,, what maybe 100 watts, TV maybe 100.

    So lets see,

    2 fans 8 hours=400 wh ?
    Clock 24 hours=120 wh
    Stereo 4 hours=400 wh
    TV 4 hours =400wh

    With no allowance for extras makes ~1.3kw hours, or ~108 ah at 12vdc One would need to charge ~130 ah/day just to stay even with the daily draw. So, if you are looking to draw down your batteries only 20% you would need batteries ~ 650 ah of batteries. If you allow yourself to draw them down to 50% you would need ~260 ah.

    Now if you wish to add in a three day reserve you would need to triple these numbers.
    So say 500 ah of batteries would require between 25 and 65 amps to put it in the recommended charging rate, somewhere ~400 watts worth of panels.

    If you use my quick rule of thumb,, 400 wh would need (if my suspect math is close).
    To get 400 wh with no reserve,, you would need ~ 200 watts of panel (MINIMUM) just to keep even. Add in a three day reserve, and 600 watts might be a good number to start thinking about.

    Just thinking out loud, with no organization,, (what else is new)!




    In any case,, this sort of illustrates a couple of things,, the first is that your 75 watts aint gonna be enough,, and your loads are not as inconsequential as you might think,

    Good luck,

    Tony
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